The field of the invention is motor drive systems for large electric mining machines such as shovels and drag lines.
Large mining machines include three separate motor drive systems. In a shovel, for example, a first motor drive serves to hoist and lower a dipper which is fastened to the end of a handle, a second motor drive serves to crowd and retract the handle with respect to a boom, and a third motor drive serves to swing a revolving frame which supports the boom. In large shovels such as those disclosed in U.S. Pat. Nos. 3,690,493, 3,708,152, 3,901,341 and 4,053,139, Ward-Leonard drive systems have typically been employed. Such drive systems include control circuits exemplified by that disclosed in U.S. Pat. No. 3,518,444 issued to D. E. Barber on June 30, 1970 and entitled "Control System for Excavating Machinery."
More recently, a.c. induction motors have been employed to drive the various mining machine motions. One such system is disclosed in U.S. Pat. No. 4,263,535 which issued on Apr. 21, 1981 and which is entitled "Motor Drive System For an Electric Mining Shovel."
Regardless of the type of motor drive system employed, power shovels and draglines are by their very nature cyclic machines which require varying amounts of electric power during each cycle. The three main motions are utilized to dig the material, lift it, transport it to an appropriate dumping site and finally to return to the bank and begin again. To accomplish this task each main motion motor must accelerate the motion machinery, perform useful work, electrically brake the drive to a stop, and/or counteract the effects of gravity one or more times each cycle of the machine.
Each motion motor consumes energy while doing useful work (i.e. digging, lifting the material to dump etc), and each motor also returns (i.e. regenerates) energy during a portion of the cycle to be utilized by other motions, pumped back into the power line or be dissipated in some manner. A typical input power vs time curve for an electric mining machine cycle is shown by the solid line in FIG. 4. Analyzing this input power curve it can be seen that the average power drawn by the machine is less than 50% of the peak power. The regenerated peak is approximately 60% of the input peak power. Thus if the power peak is considered as 100% the total power swing is 160% during each digging cycle.
This kind of power variance can cause severe problems for small power generating or distribution systems. If the generating capacity is small (i.e. a diesel driven generator) the excavator power swing will become a large proportion of the generating capacity. When this happens severe problems can occur, for example, frequent shut downs of the system and very short equipment life. To prevent this type of problem in small generating systems expensive modifications (such as huge flywheels) must be made. Because the power generators must operate at a relatively constant speed to obtain constant frequency such modifications are expensive in both first cost and operating costs.
Where the power generating capacity is large enough to absorb the power swing, problems can still occur. Many distribution systems, particularly those feeding new mines, are too small to supply both the excavators and the other loads which depend on the electrical power system. The problem in this case usually manifests itself as voltage flicker. This condition results in irritating flicker of lights or TV picture size or it can result in the tripping of critical loads. Power supply authorities are usually very reluctant to allow new mines to begin operation or existing mines to expand where this possibility exists. While this problem usually exists only for a relatively short period of time (while an alternate line is being built) it can exist for as long as three to four years. Delaying start-up or expansion of a mine for such a period of time can be economically disasterous.